Optical gas sensors often include an infrared radiation source (e.g., in the form of an incandescent lamp) which emits thermal radiation onto a gas received in a measurement cell. The gas absorbs a portion of radiation at certain wavelengths. A detector, such as a photodiode, then detects at least portions of the thermal radiation that has passed through the measurement cell and provides a corresponding signal (e.g., in the case of a photodiode, a photocurrent) from which a gas concentration in the measurement cell can be determined.
For such a measurement, the radiation source is typically turned on for a certain pulse duration (such as, for example, several 100 ms) to generate a radiation pulse. At the beginning of the pulse duration, a relatively high peak inrush current can occur. This is true, in particular, of radiation sources exhibiting PTC behavior; i.e., radiation sources which have a positive temperature coefficient. This means that an electrical resistance of the radiation source is lower at low temperatures (e.g., at room temperature at turn-on) than at higher temperatures (e.g., some time after turn-on). Radiation sources used in optical gas sensors typically exhibit such a PTC behavior.
In incandescent lamps, such unwanted peak inrush currents can occur due to the PTC behavior and can exceed the actual operating current by a factor of 8 to 10. The peak inrush currents must be provided by the power supply of the gas sensor and must, therefore, be taken into account in the design of the system. In the case of bus-powered systems, for example, this may require relatively large (and expensive) energy buffers. In battery-operated systems, high peak inrush currents can decrease the life of the battery. Therefore, a lowest possible peak inrush current is a selection criterion for users of optical gas sensors and, consequently, an important design goal for manufacturers. In a conceivable ideal case, for example, it would be desirable that no peak inrush current be perceivable at all; i.e., that the current have a rectangular waveform and that the power consumption be substantially constant over the duration of the pulse.
In some commercially available CO2 sensors, a dropping resistor is used to reduce the peak inrush current. However, this relatively inexpensive approach has the disadvantage that the dropping resistor is connected in series with the radiation source throughout the entire pulse duration and, therefore, consumes energy permanently. It is true that the higher the selected value of the dropping resistor, the lower will be the peak inrush current, but the lower will also be the energy effectively available to the radiation source for emission.
In other commercially available CO2 sensors, the radiation source is operated at constant current, whereby, inherently, peak currents do not occur. This approach has the disadvantage that the voltage at the radiation source is initially relatively low, but rises more than proportionately with time. A selectable maximum current or the pulse duration is thereby limited because otherwise the voltage at the radiation source would drive the current source into saturation. In addition, in the case of constant-current operation, the voltage, and thus the power, at the radiation source exhibit a pronounced temperature dependence. Moreover, an aging effect of the light source; i.e., an increase in (cold) resistance over the lifetime of the radiation source, is intensified by the constant-current operation because, unlike the constant-voltage operation, an increased cold resistance does not result in a smaller current and, therefore, the circuit experiences no self-regulation.
US 2016/0172855 A1 proposes to apply a constant electrical power to a resistive load, such as, for example, an IR emitter in a Fourier transform infrared (FTIR) spectrometer. In U.S. Pat. No. 5,095,270, too, constant power is supplied to an IR light source in a gas sensor. In both cases, a current sensor and an (analog) multiplier are needed to regulate the power.
U.S. Pat. No. 6,023,069 proposes to operate a radiation source at a quasi-constant power over the lifetime thereof and, for this purpose, to connect suitable resistors between a voltage source and the radiation source. However, the dynamic behavior of the radiation source is not taken into account in this approach.